The use of structural steel and reinforced concrete in construction has allowed for large buildings to be supported by a system of columns rather than their exterior walls. Curtain walls are non-structural walls placed on the exterior of multi-story buildings. The primary purpose of a curtain wall is to provide a barrier between building inhabitants and the outside elements. These curtain walls are traditionally aluminum frames filled with sheets of glass, metal, or stone. Glass is a popular choice because of the added benefit of allowing in natural light.
Curtain walls can be classified by their method of fabrication and installation into the following general categories: stick systems and unitized (also known as modular) systems. In the stick system, the curtain wall frame (mullions) and glass or opaque panels are installed and connected together piece by piece. In unitized systems, the curtain wall is composed of large units that are assembled and glazed in the factory, shipped to the site and erected on the building. Vertical and horizontal mullions of the modules mate together with the adjoining modules. Modules are generally constructed one story tall and one module wide but may incorporate multiple modules. Typical units are five to six feet wide.
Water penetration resistance is a function of glazing details, drainage details, sealants, and frame construction. Water can enter the exterior wall system by means of five different forces: gravity, kinetic energy, air pressure difference, surface tension, and capillary action. To mitigate water infiltration, all of these forces are usually accounted for in the curtain wall system design.
Unlike discontinuous windows, which are smaller units and can rely to a high degree on sill flashings to capture frame corner leakage, curtain walls cover large expanses of wall without sill flashings at each glazed opening. Water penetration of curtain wall frame corners is likely to leak to the interior and/or onto insulating glass below. Watertight frame corner construction and good glazing pocket drainage are critical for reliable water penetration resistance. Additionally, due to the reduction or lack insulation in curtain wall systems, the construction materials may conduct the heat or cold from the exterior of the building, and condensation can form as a result and may create internal weepage within the system.
Typically, curtain wall systems transfer their own dead load plus any live loads (which consist primarily of positive and negative wind loads) back to building structure or intermediate framing. In certain situations, the curtain wall system may demonstrate movement caused by thermal changes and wind significantly different than movement of the building structure. Therefore the connections to anchor the curtain wall must be designed to allow differential movement while resisting the loads applied while at the same time allow for weepage and the control of thermal transfers between the outside and inside of the building.
What is needed, therefore, is a device to act as an insulating frame and provide an effective gutter system for exterior condensation or rainwater while minimizing any weepage from reaching the interior and offering protection at the weak point of the seal.